1. Field of the Invention
The invention relates generally to the field of carrier tracking. More particularly, the invention relates to symmetrical (balanced) carrier tracking.
2. Discussion of the Related Art
Carrier tracking, or xe2x80x9cclock recoveryxe2x80x9d, is a fundamental process in all high performance receivers, regardless of whether the information is xe2x80x9canalogxe2x80x9d (such as speech, etc.) or xe2x80x9cdigitalxe2x80x9d (i.e. digital data). The receiver must establish the frequency of the transmitter""s oscillator, denoted by fCT. If fCR is the receiver""s estimate of the transmit carrier frequency, the performance degradation is directly related to the difference |fCTxe2x88x92fCR|. Thus the ability to accurately estimate the carrier frequency is important for good system performance. For a variety of reasons, such as oscillator drift, Doppler effects, and so on, the effective carrier frequency may change with time and the receiver must follow these changes. This tracking is achieved using a servo loop often referred to as the carrier tracking loop. The principal components of a carrier tracking loop include an NCO (or a VCO), the output of which is representative of the carrier frequency estimate, a means to generate an xe2x80x9cerrorxe2x80x9d signal representative of the difference (fCTxe2x88x92fCR), and a xe2x80x9cloop filterxe2x80x9d to smooth out noise from this error estimate. The loop is closed by using this smoothed error signal to drive the NCO.
A disadvantage of this approach has been relatively high cost. Therefore, what is needed is a solution that meets the above-discussed requirements in a more cost effective manner. The invention is directed to meeting these requirements, among others.
A goal of the invention is to satisfy the above-discussed requirement of economy.
One embodiment of the invention is based on a method for tracking a center frequency of a signal having substantially equal spectral power content on both sides of the center frequency, comprising: making an estimate of the center frequency; filtering the signal to produce a first output having a first spectral power content and a second output having a second spectral power content; and revising the estimate of the center frequency based on a difference between the first spectral power content and the second spectral power content.
Another embodiment of the invention is based on an apparatus for tracking a center frequency of a signal having substantially equal spectral power content on both sides of the center frequency, comprising: a first filter for producing a first output from the signal; a second filter for producing a second output from the signal, such that the second filter is matched with the first filter; and a set of processing resources for making an estimate of the center frequency, calculating the spectral power contents of the first output and the second output, and revising the estimate of the center frequency based on a difference between the spectral power contents of the first output and the second output, such that the set of processing resources is coupled to the first filter and the second filter.